Control system for controlling the outer diameter of a strand of plastic material, in particular of a cable

ABSTRACT

A system for controlling the outer diameter of a strand, particularly of a cable or the like including a measuring device for measuring the outer diameter of the strand of plastic material emerging from the extruder. The signal of the measuring device is fed to a comparator which compares it with a reference diameter and generates a difference signal. A control unit receives the difference signal to control the rotational speed of the extruder and/or the speed by which the strand emerges from the extruder. A computing stage is connected to the control unit which generates a comparison signal modulated by the actual diameter signal and fed to the control unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 07/529,139 filed on May25, 1990 now U.S. Pat. No. 5,169,649.

BACKGROUND OF THE INVENTION

The invention refers to a control system for controlling the outerdiameter of a strand of plastic material, particularly of a cable or thelike.

Usually, cables are manufactured in that a conductor is moved through anextruder which applies an insulating layer onto the conductor, thematerial of the insulating layer being suited to be vulcanized orcross-linked. After the extrusion, the cable or the strand is conveyedto a vulcanization or cross-linking path and thereafter cooled. In orderto achieve a predetermined insulation strength, the layer extruded ontothe cable has to have a minimum thickness. On the other side, thediameter of the layer is to be selected only as large as necessary tosave material and weight. For this reason, it is necessary tocontinuously monitor the diameter of the cable or the strand and tocarry out a respective control if a predetermined value is exceeded orhas not come up.

For the above reason, it is a necessity to continuously measure theouter diameter of the cable. The diameter can be changed by changing theconveying speed of the cable and/or the rotational speed of theextruder. Conventional control means compare the measured diameter witha reference value and form the difference of the signals which istransmitted to a control unit to change the conveying speed of theextruder line or the rotational speed of the extruder until thereference value has been reached. If the dead time of the system to becontrolled is relatively small, i.e. if the distance between theextruder and the device measuring the diameter is relatively small, thebehaviour of the control system can be satisfactorily adjusted to apredetermined diameter value. A disadvantage with respect to theconventional control is the fact that a satisfactory balancing can beachieved only for a specific diameter value. If the cable to bemanufactured has a smaller diameter, the control works more slowly. If alarger diameter is to be manufactured, the control works more rapidlyand therefore may become unstable.

SUMMARY OF THE INVENTION

The object of the invention is to provide a control system by which adiameter of a cable or a strand can be simply, inexpensively andprecisely controlled on accurate values independent from the quantity ofthe desired reference diameter value.

This object is attained by the invention.

In the control system according to the invention, the control unit isconnected to a computing stage which generates a difference between theactual diameter value and a reference diameter value which differencebeing dependent upon the diameter.

The invention has recognized that usually, the diameter of the cablemanufactured changes in a wider range than the ratio of the diameter ofthe conductor to the outer diameter of the cable. With respect to aconstant control behaviour, it is appropriate to use the relativediameter error as correction quantity. By means of the invention,therefore, a stable control behaviour can be achieved through a largediameter range which control behaviour, thus, is independent from thediameter. In the operation of the extruder, therefore, it is no longernecessary to adapt the control system accordingly if another diameter isdesired as usual in conventional systems wherein the adaptation is madeon the basis of experience.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit of a control system according to theinvention.

FIG. 2 is a further embodiment of the control system according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known in connection with apparatuses for the manufacturing ofcables or the like to locate a diameter measuring device between theextruder and a cooling path the latter being not accessible for themeasurement of the diameter. This arrangement has the advantage that arapid control can be achieved. However, it is disadvantageous that onlythe so-called warm diameter can be measured which differs from theso-called cold diameter through a specific shrinkage value. In themanufacturing of cables, only the cold diameter is decisive. Therefore,it is also known to locate a diameter measuring device behind thecooling path. The disadvantage of this arrangement is that the controlprocess is relatively slow. Therefore, it is desirable to provide adiameter measurement between the extruder and the cooling path which,however, is based on the actual cold diameter so that also the referencediameter can be based on a cold diameter.

For the control of the extruder or the conveying speed, the diametermeasuring device may be located adjacent the extruder, two furthermeasuring devices being provided, one of which measuring the mass flowof the extruder and the other the conveying speed. Usually, signalsrepresentative of the mass flow and the conveying speed are available inconventional production means, e.g. as a voltage of a tachometergenerator. A computing stage calculates the desired cold diameter fromthe measured warm diameter, the conveying speed and the mass flow aswell as from the known values for the density of the used thermoplasticmaterial of the strand and under consideration of the known shrinkagevalues.

Thus, a cold reference diameter can be introduced in the comparator forthe control if the comparator receives the actual cold diameter which iscalculated from the actual warm diameter in the manner described above.By means of the described process, a rapid control can be achieved underimmediate consideration of the cold diameter which is solely essential.

It has already been mentioned that for a rapid control process in theprior art, it is preferred to measure the warm diameter directlydownstream of the extruder. The measurement of the cold diameterdownstream of the cooling path can amount from some seconds to someminutes in dependence of the production means and the line speed. Bythis, the dead time of the control is negatively influenced. In theprior art, the warm diameter is usually modified by a shrinkage value tocalculate the cold diameter which calculation can be carried out withoutproblems by the use of microprocessors in process controls. Theshrinkage value is found by experience. However, this method makes itnecessary to check the actual cold value from time to time. Anembodiment of the invention, therefore, provides that a first diametermeasuring device (warm value) is located shortly behind the extruder anda second diameter measuring device is located at a distance from theextruder, preferably behind the cooling path (cold value). The diametersignal of the first measuring device is transferred to a firstcomparator means and further to a delay means, the delay time thereofcorresponding precisely to the time which the strand needs from thefirst to the second diameter measuring device. A second comparator meanscompares the diameter signal of the second measuring device with adiameter signal of the first measuring device, and the difference signalof the second comparator means is also transferred to the firstcomparator means as is the case with the reference value for the colddiameter. According to an embodiment of the invention, the delay meanscan be defined by a FIFO date memory or shift register, the shiftingfrequency being dependent upon the conveying speed. In this embodimentaccording to the invention, the actual shrinkage of the cable ismeasured due to the delay. The obtained values for the shrinkage areapproximately constant, i.e. are not substantially changed by changes ofthe diameter and do not negatively influence the control although theyare available only after the passage time between the memory stations.The advantages of a so-called warm value control, e.g. rapid reactionand elimination of disturbances are maintained. As soon as the seconddiameter measuring device delivers cold values, the control to thedesired value can be started. The produced length upon the starting of aproduction line is frequently sufficient, for example to fill the shiftregister and to detect the shrinkage so that without delay upon aswitching on of the control, a control to the desired value cancommence.

The invention is subsequently described along embodiments illustrated inaccompanying drawings.

In FIG. 1, a conductor 10 is conveyed to an extruder 11 which formsplastic material onto the conductor 10 for the production of a cable 12.The cable 12 is conveyed through cooling means 13 and conveyed towardsarrow 14 and wound by a winding mechanism not shown, the mechanismgenerating a predetermined conveying speed for cable 12. A firstdiameter measuring device 15 measures the outer diameter of the cabledirectly adjacent extruder 12. A second diameter measuring device 16measures the outer diameter of cable 12 behind path 13. The measureddiameter values are not equal since the plastic material shrinks uponcooling. A measuring device 17 generates pulses in dependence of theproduced length of cable 12.

In the following, reference is made to FIG. 2. A control unit 20 servesfor the control of the rotational speed of extruder 11 and/or theconveying speed of cable 12 (not shown in detail). A comparator 21 isfed with a reference diameter signal D_(Soll) and an actual diametersignal D_(Ist) Warm delivered by the first measuring device 15. Thedifference signal D_(Soll) -D_(Ist) is fed to a computing stage 21awhich is fed with the diameter value D_(Soll). The computing stage 21acalculates from the difference signal D_(Soll) -D_(Ist) =ΔD and thereference diameter D_(Soll) the ratio ΔD:D_(Soll). This signal is fed tocontrol unit 20. Thus, the difference between the actual and thereference diameter is modified in response to the diameter and fed tothe control unit. By this, a stable control behaviour can be achievedindependent from the outer diameter of cable 12. Instead of diameterD_(Soll), also diameter D_(Ist) can be fed to the computing stage 21a.

As an alternative to the described control operation or as an addition,a second computing state 22 can be provided. A speed signal v is fed tocomputing stage 22 which for example is derived from the pulses of themeasuring device 17. Additionally, a mass flow signal ρ is fed intocomputing stage 22 which is derived from the extruder (not shown indetail). The mass flow determines the cross-sectional annular surface ofthe insulation sheath on the conductor if the diameter and the conveyingspeed are predetermined. Finally, also the shrinkage ##EQU1## is fed tocomputing stage 22. Computing stage 22 thus converts the warm diameterinto the cold diameter by the mentioned values which is compared withthe cold reference diameter in comparator means 21. This control methodallows a rapid control on the actually intended cold diameter.

In the embodiment of FIG. 1, the shrinkage which the outer diameterexperiences between the warm and the cold state is actually determinedby the measuring devices 15, 16, each for identical cable sections. Forthis purpose,a FIFO (first-in-first-out) date memory or shift register30 is provided into which the warm diameter value D₁ is fed and shiftedthrough the shift register 30 in accordance with the cycle by which thecable 12 is moved. The diameter value D₁ ' at the exit of shift register30 is compared with the cold diameter value D₂ in the comparator means31. The difference signal is fed to comparator means 32 into which thewarm diameter D₁ and the reference diameter D₂ are fed to determine adifference which is transmitted to the control unit 33. The control unitwhich has also integral behaviour operates in the same manner as thecontrol unit 20 of the embodiment of FIG. 2. The difference signaldetermined in the comparator means 31 occasionally can be formed to amean value in a mean value stage 34 to compensate for certainfluctuations of the diameter caused by an unroundness of the cable.

We claim:
 1. A control system for controlling the cold outer diameter ofa strand of plastic material formed by an extruder, the strand having aspeed at which it is withdrawn from the extruder, the control systemcomprising a measuring device for measuring an actual outer diameter ofthe strand emerging from the extruder and generating a signalrepresentative of the actual outer diameter of the strand at themeasuring device and comparing it with a reference diameter signalrepresentative of a reference diameter value and deriving a differencesignal, a computing unit configured for receiving the difference signaland providing a modulated comparison signal, the difference signal beingmodulated either with the actual diameter signal or the referencediameter signal, and a control unit receiving said modulated comparisonsignal and responding thereto for the control of either the rotationalspeed of the extruder, or the speed by which the strand is withdrawnfrom the extruder, or both.
 2. A control system as in claim 1 whereinsaid computing unit is configured to provide said modulated comparisonsignal in the form of a signal representative of a ratio of thedifference signal to either the actual diameter signal or the referencediameter signal.
 3. An extrusion apparatus for extruding a strand ofplastic material comprising an extruder, means for withdrawing thestrand from the extruder at a predetermined speed, a cooling meanspositioned in the path of the strand after it emerges from the extruderfor cooling the strand and a control system for controlling the coldouter diameter of the strand, the control system comprising a measuringdevice located at a position between the extruder and the cooling meansfor measuring an actual outer diameter of the strand emerging from theextruder and generating a signal representative of the actual outerdiameter of the strand at the measuring device and comparing it with areferenced diameter signal representative of a reference diameter valueand deriving a difference signal, a computing unit configured forreceiving the difference signal and providing a modulated comparisonsignal, the difference signal being modulated either with the actualdiameter signal or the reference diameter signal, and a control unit forreceiving said unit modulated comparison signal and responding theretofor the control of either the rotational speed of the extruder, or thespeed by which the strand is withdrawn from the extruder, or both.
 4. Anapparatus as in claim 3 wherein said computing unit is configured toprovide said modulated comparison signal in the form of a signalrepresentative of a ratio of the difference signal to either the actualdiameter signal or the reference diameter signal.